Polytwinned microstructure in CoPt and FePt alloys is formed in the course of atomic ordering at temperatures T⩽Tcr (Tcr is the temperature of the phase transformation A1→L10). Polytwinned CoPt and FePt crystals consist of regular systems (plates, blocks, bundles of plates) of plate-like twins — crystallites with different orientations of tetragonal axes, which are called C-domains. In our work, particular part that is taken by the dispersity and the way of contacting of structure elements in the formation of magnetic domain structure (DS) and of hysteresis properties of the CoPt and FePt alloys is revealed. Magnetic DS of polytwinned crystals is shown to have a cooperative nature and to consist of magnetic micro- and macrodomains. The models of such a cooperative DS are suggested. The concept of the critical size for a polytwinned crystal particle to consist of a single magnetic macrodomain is introduced. The quantitative relation between the main parameters of the cooperative DS (the width of magnetic macrodomains and the critical size of being a single macrodomain) and the C-domain thickness and the dimensions of plates and bundles is theoretically investigated. The influence of exchange and magnetostatic interactions that arise at twin boundaries of C-domains on the DS parameters is discussed. The results of numerical calculations of hysteresis properties with allowance made for these interactions are presented. A conclusion is made that the exchange and magnetostatic interactions in a twinned system of plate-like crystallites with high uniaxial anisotropy create additional metastable magnetic states, which effectively causes the polytwinned crystals to be magnetically multiaxial.